This invention relates to an electronic system which controls current from a high voltage, high power source in response to a low input signal, and thereby produces as an output an amplification of the input signal.
Many electronic devices are available for control of current to a load in response to a low input signal, such as an electron tube or transistor. Most have the ability to produce an output at all times proportionate to its input over a substantial range. Transistors are, of course, the preferred devices because of their low power losses. However, their use to control high voltage, high power systems has been limited. For example, when such an electronic control system is connected to a solar array for testing, such as to plot its current-voltage (I-V) characteristic curve, the open circuit voltage (V.sub.oc) may be over 300 volts, and the short circuit current (I.sub.sc) may be over two amperes. This is well over the rated maximum of most commercially available electronic load control systems.
Although high voltage power transistors are readily available, problems of second breakdown (in bipolar transistors) and power dissipation limit any single transistor to operation at relatively low currents when at high voltage or low voltages when at high current. Thus, a single transistor may be satisfactory for switching between these two states, but cannot carry a sustained high current at high voltages. Use of high voltage bipolar transistors to control the load requires relatively large numbers of parallel current paths, and great care in matching these currents, to prevent current hogging and second breakdown destruction of the transistors.
Power MOSFET's with high voltage ratings and reduced on-resistance have now become available. These transistors are immune to second breakdown, have larger safe operating areas of their current-voltage characteristic curves and are easy to operate in parallel. However, to stay within their safe operating region, it is still necessary to use multiple parallel current paths or current limiting resistors in series with the MOSFET's, except at lower voltages. These resistors would prevent the continuation of the I-V curve down to zero voltage because of the voltage drop across them. The present invention provides an arrangement of parallel current paths which overcomes this deficiency, permitting these parallel current paths to share the current load from open circuit (V.sub.oc) to short circuit (V.sub.sc).